CD147 is involved in many physiological functions, such as lymphocyte responsiveness, spermatogenesis, implantation, fertilization and neurological functions at early stages of development. CD147 is a member of the immunoglobulin family of receptors. Members of this family play a role in intercellular communication involved in many immune-related functions, differentiation and development. CD147 plays a role in spermatogenesis, lymphocyte activation, expression of monocarboxylate transportcrs (MCT) and has been identified as a regulatory subunit of the γ-secretase complex in Alzheimer's disease amyloid β-peptide production (Gabison and Mourah Connect Tissue Res. 49:175-179, 2008; Iacono et al. Exp. Mol. Pathol. 83:283-295, 2007; Nabeshima et al. Pathol. Int. 56:359-367, 2006; Gabison et al. Biochimie 87:361-368, 2005; and Muramatsu et al. Histol. Histopathol. 18: 981-987, 2003).
Some of these insights were obtained from the study of cd147−/− mice. These animals are defective in matrix metalloproteinase (MMP) regulation, spermatogenesis, lymphocyte responsiveness and neurological functions at the early stages of development. Such female mice are infertile due to failure of implantation and fertilization (Muramatsu et al. Histol. Histopathol. 18: 981-987, 2003). CD147 is involved in the transport of the MCT-1 and MCT-3 to the plasma membrane since reduced accumulation of these transporters has been observed in the retina of cd147 knockout mice. A functional role of CD147 in cell adhesion is supported by its involvement in the blood-brain barrier and its interactions with integrins.
CD147 has been implicated in many pathological processes, such as rheumatoid arthritis, experimental lung injury, atherosclerosis, chronic liver disease induced by hepatitis C virus, ischemic myocardial injury and heart failure (Gabison et al. Biochimie 87:361-368, 2005). Treatment of transplant patients with a CD147 antibody was effective due to inhibition of T-cell activation (Deeg et al., Blood 98:2052-2058, 2001).
CD147, a transmembrane protein of the immunoglobulin (Ig) superfamily was identified independently in different species and has many designations across different species such as M6, Neurothelin, 5A11, HT7, OX-47, CE9, EMMPRIN, Basigin, and gp42 (Kasinrerk et al. J. Immunol. 149:847-854, 1992; Altruda et al. Gene 85:445-451, 1989; Miyauchi et al. J. Biochem. 107:316-323, 1990; Seulberger et al. EMBO J. 9:2151-2158, 1990; and Fossum et al. Eur. J. Immunol. 21:671-679, 1991). The most prevalent standard isoform is a single-chain type I transmembrane molecule composed of a 21 amino acid signal sequence, a 186 residues-long extracellular domain consisting of two Ig-like domains, a transmembrane domain of 21 amino acids and a cytoplasmic domain of 41 residues.
The transmembrane region harbors a leucine zipper and a charged residue (glutamic acid). The corresponding gene is located on chromosome 19p13.3 and encodes a 29 kDa backbone protein. Three N glycosylation sites have been identified and migration on sodium dodecyl-sulfate polyacrylamide gel electrophoresis (SDS-PAGE) occurs between 39 and 65 kDa depending on the degree of glycosylation.
CD147 has a broad expression pattern on hematopoietic and non-hematopoietic cells such as monocytes, granulocytes, epithelial and endothelial cells. Weak expression has been noted on resting T lymphocytes, whereas expression is increased on activated T lymphocytes and monocytes (Kasinrerk et al. J. Immunol. 149:847-854, 1992; Altruda et al. Gene 85:445-451, 1989; Miyauchi et al. J. Biochem. 107:316-323, 1990; Seulberger et al. EMBO J. 9:2151-2158, 1990; Fossum et al. Eur. J. Immunol. 21:671-679, 1991; and Ochrietor et al. Invest. Ophthalmol. Vis. Sci. 44:4086-4096, 2003). The perfect conservation of the amino acid sequences of the transmembrane sequences across all listed species (Iacono et al. Exp. Mol. Pathol. 83:283-295, 2007) as described above is a remarkable feature, which includes the presence of a conserved glutamic acid residue. This finding indicates the involvement of transmembrane amino acids in protein-protein interactions within the plasma membrane. The cytoplasmic domains are more conserved than the extracellular domains pointing to similar considerations with respect to conserved protein-protein interactions with proteins located in the cytoplasm. Inhibition of CD147 expression by RNAi led to significantly decreased angiogenesis in vitro. CD147 may regulate angiogenesis by several mechanisms including proliferation, survival, MMP secretion and phosphoinositide 3 kinase/protein kinase B (PI3K/Akt) activation.
In several cancer cell lines, CD147 has been identified as a mediator of anti-apoptotic function and chemoresistance. In HO-8910 ovarian carcinoma cells, CD147 RNAi reduces tumor cell invasion, tumorigenicity and chemosensitivity to paclitaxel (Zou et al. Cancer Lett. 248: 211-218, 2007). Up-regulation of CD147 has been observed in several multidrug-resistant cancer cell lines (Toole et al. Drug Resist. Update 11:110-121, 2008).
Independently, involvement of CD147 in resistance of cancer cells to a variety of chemotherapeutic agents was reported (Li et al. Cell. Mol. Life Sci. 66:504-515, 2009). In addition, CD147 was identified as a receptor which promotes androgen-independent growth of tumor cells in a hyaluronan-dependent manner (Marieb et al. Cancer Res. 64:1229-1232, 2004). In human oral squamous carcinoma cells (SCC), CD147-directed RNAi reduced X-chromosome linked inhibitor of apoptosis protein (XIAP) expression and increased chemosensitivity to 5 fluorouracil (Kuang et al. Cancer Lett. 276:189-195, 2009). In breast cancer cell lines, it was shown that CD147 confers resistance to anoikis as demonstrated by activation of caspase.
Many mAbs directed against CD147 interacting with distinct epitopes have been established (Koch et al. Int. Immunol. 11:777-786, 1999). Most of the antibodies only bind to phytohemagglutinin (PHA) stimulated T-cells, not to resting T-cells. This phenomenon was explained by bivalent binding of the low-affinity antibodies to clustered CD147 molecules on activated T-cells and not by neoepitopes specifically displayed on activated T-cells. High affinity antibodies were able to bind in a monovalent fashion to resting T-cells, which are low expressors of CD147.
Induction of dimerisation by the low-affinity antibodies resulted in inhibition of CD3-mediated T-cell activation. High-affinity mAb MEM-M6/6, recognizing a unique epitope, inhibits T cell activation by 80% and as outlined in a previous section also proliferation of colon cancer and melanoma cells and not non-transformed fibroblasts (Baba et al. Biochem. Biophys. Res. Commun. 374:111-116, 2008). Triggering of CD147 by mAbs was shown to cause displacement of glycosylphosphatidylinositol (GPI)-anchored co-receptors CD48 and CD59 from microdomains in human T-lymphocytes (Staffler et al. J. Immunol. 171:1707-1714, 2003). Perturbation of microdomains is responsible for inhibition of T-cell proliferation. Making use of COS-7 transfectants and mAbs covering different CD147 epitopes, it was shown that CD147 contains different epitopes involved in regulation of cell adhesion (homotypic cell aggregation) and lymphocyte activation (Chiampanichayakul et al. Immunobiology 211:167-78, 2006).
A different set of CD147-directed mAbs were evaluated with respect to treatment of hepatocellular carcinoma (HCC) (Xu et al. Mol. Cancer Res. 5:605-614, 2007). mAb Hb18G and Licartin, a 131I-labeled F(ab′)2 fragment of mAb Hb18G) mediate suppression of MMP secretion in cocultured fibroblasts and inhibit invasion, and Licartin significantly inhibited the growth of HCC cells. mAb Hb18G and Licartin effectively reduced growth and metastasis as well as the expression of stromal factors such as MMPs, VEGF and fibroblast surface protein (Xu et al. Mol. Cancer Res. 5:605-614, 2007). Clinical studies were reported for targeted radioimmunotherapy of HCC patients with Licartin (Xu et al. Hepatology 45:269-276, 2007). Of the 73 patients completing two cycles in a phase II trial, 6 (8%) were noted to have a partial response, 14 (19%) a minor response and 43 (59%) had stable disease. The survival of progression-free patients was significantly higher than that of patients with progressive disease. Chimeric CD147 antibody (IgG1) referred to as CNTO 3899 was evaluated as an agent for potential treatment of head-and-neck squamous cell carcinoma (Dean et al. Clin. Cancer Res. 15:4058-4065, 2009). The antibody inhibited proliferation of SSC-1 and FaDu cells up to 57%. Inhibition of collagen degradation was noted as well. Significant tumor growth inhibition was noted in SSC-1 xenografts. CNTO 3899 augments radiation response of SSC-1 and FaDu cells in vitro and in xenografts. Furthermore, the same study showed that CD147 function is associated with cytokine production of proinflammatory and proangiogenic factors such as IL1β, IL6, IL8 and VEGF. Inhibition of cytokines, MMPs and VEGF seems to mediate the mechanism of action of this mAb. The studies as outlined would suggest that inhibition of proliferation of tumor cells by CD147 mAbs in the absence of immune effector cells is either cell-type-specific and/or might be dependent on distinct epitopes the mAbs are directed against. Inhibition of T-cell activation and/or depletion was not investigated with CNTO 3899, Hb18G or Licartin.